The invention relates to a process for preparing 1,1-dihalo-1,2,2,2-tetrafluoroethanes by isomerization of 1,2-dihalotetrafluoroethanes, catalyzed by active complexes obtained "in situ" from aluminium halides, in particular from aluminium bromide or chloride. It is known, for example from M. Hudlicky, Chemistry of Organic Fluorine Comp., 2nd Ed., John Wiley, 501-2 (1976), that fluororalkanes such as CFCl.sub.2 --CCl.sub.2 F, CF.sub.2 Cl--CCl.sub.2 F, CF.sub.2 Br--CHClF, CF.sub.2 Br--CFClBr, CF.sub.2 Cl--CHFI, may undergo a molecular re-arrangement reaction catalyzed by aluminium halides.
However, the isomerization of 1,2-dihalotetrafluoroethanes into the corresponding asymmetric compounds 1,1-dihalo-1,2,2,2-tetrafluoroethanes has not been described so far.
It is only known, from D. J. Burton and L. J. Kehoe, J.O.C. 35, No. 5, 1339-41 (1970), that 1,2-dibromoterafluoroethane undergoes a dismutation by treatment with anhydrous aluminium chloride, under reflux conditions, leading to CF.sub.3 --CBr.sub.3, evidently accompanied by an equivalent amount of CF.sub.3 --CF.sub.2 Br.
Actually, we have ascertained that anhydrous aluminium chloride, when brought into contact with CF.sub.2 Br--CF.sub.2 Br, does not promote the isomerization thereof, even if the reaction mixture is heated at reflux for many hours.
On the other hand, when employing, as a catalyst for isomerizing CF.sub.2 Br--CF.sub.2 Br, anhydrous aluminium bromide, which is generally known as the most efficient out of the two Lewis acids, we have observed that the reaction which develops is not reproducible, involves long and too variable induction times and has a non-controllable trend.
In particular we have ascertained that, if the contact between anhydrous AlBr.sub.3 and CF.sub.2 Br--CF.sub.2 Br is accomplished at room temperature, no reaction occurs in a time-period of a few hours, while after one or more days CF.sub.2 Br--CF.sub.2 Br results, in many tests, to be completely dismutated with formation of CF.sub.3 --CBr.sub.3 as a solid residue.
Conversely, if the reaction mixture is brought to and maintained under reflux conditions, after highly variable induction times, from 4 hours to many hours, a very intense reaction sets in, which has a non-controllable trend, with forming of variable amounts of isomer CF.sub.3 --CFBr.sub.2 accompanied by amounts of CF.sub.3 --CBr.sub.3 and of C.sub.2 F.sub.5 Br gradually increasing as the reaction proceeds.
This non-reproducible trend is particularly dangerous when the reaction is accomplished on a scale higher than 10 Kg, as the high exothermy involved could lead to a violent course of the reaction with overflow of material from the cooler.
In no case have we succeeded in obtaining a CF.sub.3 --CFBr.sub.2 yield higher than 70%.
Therefore, a method of preparing, on a commercial scale, CF.sub.3 --CFBr.sub.2 by isomerization of CF.sub.2 Br--CF.sub.2 Br, reproducible and controllable in sufficiently safe conditions, was neither available nor inferable from the prior art.